This invention relates to high-speed differential serial link cable connectors. More specifically, this invention relates to suppression of common electrical noise on the cable braid for the reduction of electromagnetic emissions using technique applicable to the printed circuit board (PCB) on which this cable connector is mounted.
The operation of high-speed serial data connectors between peripheral devices, for example, a server or a desktop computer transmits data at high speeds between various peripheral/system devices. Other examples of such a high-speed serial differential connector usage includes connecting two peripheral devices through a high-speed serial differential connection. These peripheral devices can be, but are not limited to, a memory box or a redundant array of independent disks (RAID) controller. The cable connector in high-speed serial applications requires special shielding enclosures to isolate board-level noise from the external environment.
In general, the high-speed serial differential connector (HSSDC) metallic enclosure is connected to the chassis and the outer braid of the double shield (e.g. skew clear) cable. For safety reasons, the Transistor-to-Transistor Logic (TTL) or Logic ground is not connected directly to chassis ground. One such safety reason is that when two system/devices separated by a sufficient distance can have ground at different potential, a sizeable amount of current can flow along the cable from the one chassis to the second chassis. The potential difference can be very large and the cable provides a low impedance path, and hence large current on the cable shield. In such case, if the chassis is connected to the logic grounds of two systems, through the connector shell to the shell of another device which is at a different ground state/potential, the logic circuit at the low and high-potential ends may be burned out. In such a configuration, the two ends of the cables may have a voltage potential difference, particularly if each end of the cable attaches to a peripheral device that is at a different local ground potential. As a general practice, a high value resistance between the connector enclosure/chassis (connected to the local chassis ground) and the logic ground is recommended to provide a direct current (DC) path for the electrical current. This resistance provides a low current DC path for any voltage potential difference between the two ends of the cables.
For high-frequency data transfer applications, typically leaving the connector outer shell floating with respect to the logic ground allows a large amount of high-frequency surface currents on the connector shell. The surface current can get transferred to the outer braid of the cable and radiate. Providing a complete metallic closure of all the slots between the outer shell of the connector to the bracket, for example a peripheral component interconnect (PCI) bracket, for the chassis can significantly reduce the amount of common-mode current transferred to the outer braid of the cable. This technique of electromagnetic emission reduction is typically difficult to implement from a cost perspective in a large volume-manufacturing environment.
Another commonly used board level implementation technique to deal with the suppression of common mode noise (CMN) of the system on cable connectors (and hence the cable braid) is usage of an isolation transformer. The isolation transformer is connected in series with the differential signal lines of the cable and mounted on the PCB board near the ends of the cable connectors that connect to the peripheral devices. Isolation transformers reject/block common mode noise, and only allow the differential mode signal to pass into the differential signal pairs of the cable connector assembly. During this process the isolation transformer also attenuates the differential signal, which is not a severe problem for low data rate transfer applications. However as the frequency of signal increases, the isolation transformers degrade signal characteristics such as rise time and levels. At high frequencies, due to electrical parasitics, the isolation transformers have severe frequency-dependent attenuation characteristics and hence lead to the severe deterioration of differential signal quality due to inter-symbol interference (ISI). In general, the isolation transformer is adequate to reduce the electromagnetic interference around 200-300 MHz but has limited applications above one GHz.